Medieval scholastics share a commitment to a substance-accident ontology and to an analysis of efficient causation in which agents act in virtue of their powers. Given these commitments, it seems ready-made which entities are the agents or powers: substances are agents and their accidents powers. William of Ockham, however, offers a rather different analysis concerning material substances and their essential powers, which this article explores. The article first examines Ockham’s account of propria and his reasons for claiming that a material (...) substance is essentially powerful sine accidentibus. However, the article subsequently argues that, given Ockham’s reductionism about material substance, only substantial forms – never substances – are truly agents and powers. Thus, a material substance is essentially powerful but only by courtesy – per accidens, as Ockham calls it – because it has a non-identical part, its substantial form, which does all the causal work by itself, per se. (shrink)